Matrix storage and dispensing system

ABSTRACT

The present invention provides a system and process providing variable access to, as well as quick and accurate dispensing of, numerous selected reagents from a mass storage arrangement. According to one embodiment, an array of reagent dispensers is supported over a movable platform assembly. The platform assembly aligns a designated receiving receptacle under a selected dispenser of the array so that a respective reagent can be dispensed therein. Advantageously, the apparatus and process can be carried out under the control of a programmed computer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/955,554, filed Sep. 18, 2001, which in turn is a continuation of U.S.patent application Ser. No. 09/251,232, filed Feb. 16, 1999, now U.S.Pat. No. 6,432,719. The complete disclosures of the above referencedpatent applications are incorporated herein by reference in theirentirities.

FIELD OF THE INVENTION

The present invention relates to the storage and dispensing ofsubstances. More particularly, the invention provides a system, andmethod of use, for serially dispensing a large number of reagents into aplurality of receptacles.

BACKGROUND OF THE INVENTION

In chemical and biological laboratories, reagent transfer from a sourcevessel to a target receptacle is a fundamental task. Typically, atechnician must retrieve various reagent bottles from a storagelocation, each containing a substance pertinent to the task at hand. Thetechnician then manually pipettes a precise quantity of each into anappropriate reaction receptacle, such as a selected well of a multi-wellplate. To prevent contamination, the pipette tip must be cleaned aftercontact with each different reagent, or it must be discarded andreplaced with a new tip.

Alternatively, the technician can attempt to manually pour each of thecollected reagents from its storage vessel into a desired reactionreceptacle. However, given the ultra-small quantities of reagentstypically called for in modern-day protocols, particularly for expensivereagents, this technique can be very tedious and difficult to accuratelyperform. Moreover, the act of pouring often leads to wasted reagent,e.g., where excessive amounts are inadvertently dispensed, andcross-contamination between receptacles can result, especially whenworking in a high-density receptacle format (e.g., a plate or trayhaving ninety-six wells).

Thus, it is not surprising that such manual techniques fail to meet thedemands of most laboratories, where very small quantities of numerous(e.g., hundreds or thousands) reagents must be dispensed in a quick andaccurate manner.

While systems are known that automate certain aspects of reagentstorage, retrieval and/or dispensing, these too are associated withcertain disadvantages. One such system, available from Sagian Inc.(Indianapolis, Ind.), automates the picking and placing of reagents.Briefly, to “pick” a reagent is to retrieve it from a reagent file, andto “place” it is to re-file it back into the reagent file. The Sagiansystem employs two industrial robots to move reagents to and from anoperator area. The first robot is, a mini-trieve that moves to avertical file holding a target reagent and then pulls out an appropriatedrawer containing the reagent. The robot then delivers the drawer toanother work area where a CRS articulated robot removes the requestedreagent, verifies that it is the correct container by passing thecontainer in front of a bar-code scanner, and places it into one of aseries of racks which are accessible by the operator. The mini-trievethen returns the drawer to its original location in the file. Whileeliminating much of the labor burden and handling errors generallyassociated with manual techniques, manual intervention is neverthelessrequired in order to dispense the reagent. Moreover, much wasted effortis involved since each drawer retrieved by the robot usually containshundreds of additional reagents that do not pertain to the task at hand.Further, the robotic motions involved, and distances traversed, inretrieving each reagent can be quite substantial. Cumulatively, theoverall process can be quite time consuming, particularly in situationswhere a great number of reagents (e.g., hundreds or thousands) must beretrieved.

Another automated system is sold under the trade name HAYSTACK,available from The Automation Partnership Group pic (Melbourn SciencePark, Melbourn, Royston, Hertfordshire, UK). Similar to the Sagiansystem, the HAYSTACK system utilizes industrial robots to retrievedrawers of reagents from vertical files. In addition to suchpick-and-place functions, The Automation Partnership offers modules thatare able to carry out various dispensing steps. Such added capability,however, substantially increases the operational complexity of thesystem, and can consume a great deal of valuable laboratory space, aswell.

There is, thus, a need for a relatively simple and compact reagentstorage and dispensing system that provides for variable (custom)retrieval, as well as quick and accurate dispensing, of numerousselected reagents.

SUMMARY OF THE INVENTION

In one of its aspects, the present invention provides a system forstoring and dispensing a plurality of reagents.

According to one embodiment, the system includes an addressable array ofreagent dispensers, each having a gate mechanism disposed at a loweroutlet region thereof. The gate mechanisms are independently operablebetween (i) an opened condition permitting passage of a respectivereagent through the outlet region, and (ii) a closed condition whereatsuch passage is blocked. A first support is disposed below the dispenserarray, and a second support, having a holding area for receiving aplurality of receptacles, is mounted on the first support. The first andsecond supports can be, for example, independently operable xy stages.The first support is variably positionable in a manner permittingplacement of a fixed target region thereof directly under any selectedone of the dispensers in the array. The second support is variablypositionable in a fashion permitting placement of any selected targetsite of the receptacle-holding area directly over the fixed targetregion.

Each of the dispensers can be, for example, an elongated containerhaving a longitudinally extending passageway configured to receive andhold a respective reagent when the gate mechanism is in the closedcondition.

A rack having an array of holding cells can support the containers.According to one embodiment, the rack has at least 100 holding cells,and preferably in excess of 1,000 holding cells. Exemplary racksinclude, for example, 5,000, 10,000, 50,000, 100,000 and 500,000 holdingcells. Each holding cell can be configured to removably support one ofthe containers in a substantially upright fashion. The holding cells canbe configured to hold the containers at an average density, for example,of between about 2-8 containers per cm², or higher. In one embodiment,the containers are disposed in the rack at an average density of betweenabout 3-6 containers per cm²; and preferably between about 4-5containers per cm². Multiple racks (e.g., 2, 3, 4, 5, or more) can bearranged in tandem for use in an “assembly line” type fashion.

A plurality of different reagents can be disposed in the dispensers. Inone embodiment, each dispenser contains a reagent that is unique to thearray.

Beads can be employed to carry the different reagents. One embodiment ofthe invention provides a plurality of bead groups, or “lots,” with eachlot being comprised of substantially similar beads carrying a respectiveone of the different reagents. The beads can be relatively large, e.g.,about 1-5 mm in diameter; or the beads can be relatively small, e.g.,each having a diameter of less than about a millimeter. In one preferredembodiment, each bead has a diameter of between about 275-325 μm; andpreferably about 300 μm.

In one embodiment, a plurality of reagent-carrying beads are held insealed ampules. In an exemplary arrangement, the ampules are dimensionedto move downward through a dispenser passageway under the force ofgravity, in a substantially single-file fashion. Preferably, all of thebeads in any given ampule carry the same, or a substantially similar,kind of reagent. Further in this embodiment, each passageway of thedispenser array is loaded with a plurality of such ampules.

One embodiment of the invention provides a detection assembly adapted todetect the passage of reagent dispensed from any one of the dispensersin the array. To this end, the detection assembly is provided with afield of view extending between the dispenser outlet regions and thesecond support.

According to one particular embodiment, the detection assembly includesa radiation emitter, such as a diode laser, and a radiation sensor. Inan exemplary arrangement, the radiation emitter is (a) mounted on thefirst support at a region along one side of the second support, and (b)configured to project a substantially linear radiation beam along apathway that passes over the fixed target region of the first support.The radiation sensor can be (a) mounted on the first support at a regionalong an opposing side of the second support, and (b) disposed withinthe radiation-beam pathway.

In one embodiment, each gate mechanism of the array is subject to abiasing force that normally urges it to the closed position, therebypreventing the passage of reagent through a respective outlet region. Arelease mechanism, adapted for positioning near any one of the gatemechanisms, is operable to apply a secondary force of a magnitude anddirection effective to override the normal biasing force so that thegate mechanism assumes the opened condition.

In one particular embodiment, each gate mechanism includes a magneticpinch valve having first and second permanent magnets that are pivotallymounted in facing relation at a respective outlet region. The magnetshave lower, confronting north and south pole regions, respectively, thatare normally urged toward one another by magnetic forces so as to pivotthe magnets to the closed condition. Further in this embodiment, therelease mechanism can be an electromagnet operable to generate amagnetic force having south and north pole portions disposed to attractthe north and south pole lower regions of the first and second pivotalmagnets, respectively, so that they swing away from one another (i.e.,to an open condition).

In another particular embodiment, each gate mechanism is a resilientlydeflectable lever having a protrusion normally extending into arespective outlet region. Further in this embodiment, the releasemechanism is a rod adapted for reciprocal linear motion between aretracted position and an extended position. Upon movement toward theextended position, the rod can mechanically engage and deflect thelever, so that the protrusion is at least partially withdrawn from theoutlet region (i.e., to an open condition).

The system of the invention can further include a guide or funnel memberlocated over the fixed target region of the first support, between thedispenser array and the second support. In a preferred embodiment, theguide member is disposed for movement with the first support to aposition under any selected dispenser. The guide member is configured tochannel reagent dispensed from such dispenser to a selected site on theholding area of the second support.

In one particular embodiment, the guide member includes (i) an upperopening, or inlet, that is alignable with any one of the outlet regionsfor receiving reagent dispensed therefrom, and (ii) a lower opening, oroutlet, through which dispensed reagent may egress in route to theholding area. Preferably, the upper opening is larger than the loweropening. A conical portion can be provided between the upper and loweropenings.

In another of its aspects, the present invention provides a reagentdispenser assembly.

According to one preferred embodiment, the reagent dispenser assemblyincludes a container adapted to receive a reagent and a gate mechanismlocated at a lower outlet region of the container. The gate mechanism isprovided with first and second permanent magnets pivotally mounted infacing relation at the lower outlet region. The pivotal magnets havelower, confronting north and south pole regions, respectively, that arenormally urged toward one another by magnetic forces so as to swing themto a closed condition whereat the egression of reagent from thecontainer is substantially blocked.

In one embodiment, an electromagnet is disposed below the gatemechanism. In this embodiment, the electromagnet is operable to generatea magnetic force having south and north pole portions disposed toattract the north and south pole lower regions of the first and secondmagnets, respectively, so that these regions swing away from one anotherto an opened condition. In this opened condition, the egression ofreagent from the container is permitted.

Another embodiment provides a rack holding a plurality of the containersat respective locations defining an array. A first movable support isdisposed below the rack, upon which the electromagnetic can be mounted.

A second movable support can be mounted on the first movable support,under the electromagnet. In this embodiment, the second movable supportis configured to receive and hold a multi-well plate for receivingreagents dispensed from the containers.

Still a further aspect of the present invention provides a method forloading a plurality of receptacles with one or more reagents.

According to one embodiment, the method includes the steps of

(i) placing the receptacles on a support under an addressable array ofreagent dispensers;

(ii) selecting a dispenser equipped to dispense a desired reagent, and areceptacle for receiving the desired reagent;

(iii) simultaneously (a) positioning a fixed target region of thesupport at a location under the selected dispenser, and (b) positioningthe selected receptacle at a location directly over the fixed targetregion of the support;

(iv) dispensing the desired reagent from the selected dispenser into theselected receptacle;

(v) detecting the desired reagent as it is dispensed from the selecteddispenser; and

(vi) repeating steps (ii)-(v) so that reagent is dispensed from at leastone other dispenser into at least one other receptacle.

In one embodiment, each of the receptacles is a well of a multi-welltray.

In another embodiment, each of the dispensers is equipped to dispense ananalyte-specific reagent that is unique to the array.

In a further embodiment, at least 100 different analyte-specificreagents are dispensed from respective dispensers into respectivereceptacles. Other embodiments contemplate the dispensing of at least500, 1,000, and 10,000, different reagents.

These and other features and advantages of the present invention willbecome clear from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure and manner of operation of the invention, together withthe further objects and advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a perspective view of a reagent storage and dispensing system,showing a dispenser poised for insertion into a holding cell of asupport rack, according to an embodiment of the present invention.

FIG. 2 is a perspective view, with portions broken away, showingadditional details of the reagent storage and dispensing system of FIG.1.

FIG. 3 is an exploded view showing still further details of the reagentstorage and dispensing system of FIGS. 1 and 2.

FIG. 4 is a partial side-sectional view of a dispenser of the inventionholding a plurality of reagent-containing ampules.

FIGS. 5(A) and 5(B) are vertical and horizontal cross-sectional views,respectively, showing a magnetic pinch valve blocking the passage ofreagent beads from a dispenser, in accordance with one embodiment of thepresent invention.

FIGS. 6(A) and 6(B) are vertical and horizontal cross-sectional views,respectively, showing an electromagnet inducing the magnetic pinch valveof FIGS. 5(A) and 5(B) to permit the passage of reagent beads.

FIGS. 7(A) and 7(B) are vertical and horizontal cross-sectional views,respectively, showing a magnetic pinch valve blocking the passage of afluidic reagent from a dispenser, in accordance with a furtherembodiment of the present invention.

FIGS. 8(A) and 8(B) are vertical and horizontal cross-sectional views,respectively, showing an electromagnet inducing the magnetic pinch valveof FIGS. 7(A) and 7(B) to permit the passage of an aliquot of fluidicreagent.

FIG. 9(A) is a side cross-sectional view showing a spring-biased leverblocking the passage of reagent-carrying ampules from a dispenser, inaccordance with one embodiment of the present invention.

FIG. 9(B) is a side cross-sectional view showing a rod-like actuatordeflecting the spring-biased lever of FIG. 9(A), so that a singlereagent-carrying ampule can fall from the dispenser into an underlyingguide or funnel member.

FIG. 10 is a perspective view of the reagent storage and dispensingsystem of the present invention in the context of a larger system forloading microcard wells with reagent-carrying beads.

DETAILED DESCRIPTION OF THE INVENTION

The following discussion of the preferred embodiments of the presentinvention is merely exemplary in nature. Accordingly, this discussion isin no way intended to limit the scope of the invention.

One aspect of the invention provides a system for dispensing a pluralityof reagents. With initial reference to the embodiment of FIGS. 1-3, thesystem generally includes a movable table or platform assembly, denotedas 12, disposed under an addressable array of reagent dispensers, as at16, equipped to serially dispense a plurality of reagents. As usedherein, the wording “addressable array” refers to an array having aknown reagent associated with a known location (address) in the array.

Platform assembly 12 includes an upper support 22 mounted on a lowersupport 26. Lower support 26 is movable such that a fixed (i.e.,constant) target or reference region thereof, e.g., as indicated at 26 ain the exploded view of FIG. 3, can be positioned below any selecteddispenser of array 16. Upper support 22 is movable such that anyselected (i.e., variable) target site of a receptacle-holding areathereof, visible as stippled region 22 a in the embodiment of FIG. 3,can be positioned over the fixed target region of lower support 26.

Briefly, in operation, the fixed target region of the lower support ispositioned under a dispenser holding a desired reagent. At the sametime, a selected target site of the upper support's receptacle-holdingarea is positioned over the lower support's fixed target region.Typically, a particular receptacle held in a specific place on thereceptacle-holding area, such as a well of multi-well plate 36, will besituated over the selected target site. Dispensed reagent, then, willfall toward the selected target site, landing in the receptacle. Thisprocedure can be repeated to load other selected receptacles withdesired reagents.

More particularly, the upper and lower supports, which can be xypositioners, e.g., stages, tables or similar devices, are adapted forvariable positioning along respective, generally horizontal planes. Suchpositioning can be effected using automated means, e.g., motorizedassemblies, or it can be manually effected. In one preferred embodiment,each of two xy stages is disposed in mechanical communication with arespective computer-controlled stepper motor (not shown) via arespective screw arrangement. Suitable xy stages and controllers areavailable commercially, for example, from NSK Inc. of Japan.

A control computer (not shown) integrates the operation of the stages,for example through a program written in an event driven language suchas LABVIEW® or LABWINDOWS® (National Instruments Corp., Austin, Tex.).In particular, the LABVIEW software provides a high level graphicalprogramming environment for controlling instruments. U.S. Pat. Nos.4,901,221; 4,914,568; 5,291,587; 5,301,301; 5,301,336; and 5,481,741(each expressly incorporated herein by reference) disclose variousaspects of the LABVIEW graphical programming and development system. Thegraphical programming environment disclosed in these patents allows auser to define programs or routines by block diagrams, or “virtualinstruments.” As this is done, machine language instructions areautomatically constructed which characterize an execution procedurecorresponding to the displayed procedure.

Interface cards for communicating the computer with the motorcontrollers are also available commercially, e.g., from NationalInstruments Corp.

The receptacle-holding area of the upper support is adapted to removablysupport a plurality of receptacles for receiving respective reagentsfrom the dispenser array. Along the holding area, means are provided formaintaining each receptacle in a desired location while the support ismoved from one place to another. For example, a slightly recessedtrough-like region can extend below the uppermost surface of thesupport, into which the receptacles can be placed. Alternatively, or inaddition, mechanical holding means such as clips, brackets, bumpers,framing, VELCRO®, or the like, and/or magnetic holding means, such asmagnetic strips on the holding-area surface and a magneticallyattractable undersurface on the receptacles, or the like, can beemployed to maintain the containers in place.

In the embodiment of FIGS. 1-3, the receptacles are provided as an arrayof spaced-apart receiving wells, such as wells 32, formed in a tray orplate 36. Each of wells 32 has an opening at its upper end, permittingthe well to receive and hold a reagent dispensed from above. Aspring-loaded plate holder (not shown), attached to the upper surface ofsupport 22 on opposing sides of holding area 22 a, prevents plate 36from sliding across upper support 22 as it is moved.

As previously indicated, lower support 26 is provided with a fixed(constant) reference or target region, such as area 26 a visible in FIG.3. The fixed target region is a specific portion of the lower support(i) that is positionable under any dispenser of the dispenser array, and(ii) over which any selected (variable) site of the receptacle-holdingarea can be positioned. Typically, placement of the fixed target regionwill be determined by the presence of one or more elements, discussedbelow, each having a position and/or operational range of motion that issubstantially fixed above a particular area of the lower support's uppersurface. For example, the fixed target region can lie under a radiationbeam, such as beam 38 in FIGS. 1-3, projectable over the receptacleholding area for detecting the passage of reagent from an overheaddispenser into a receptacle. As another example, the fixed target regioncan be located below a guide or funnel member, as at 40 in FIGS. 2-3,for channeling reagent dispensed from an overhead dispenser to aselected site on the receptacle holding area. As yet a further example,the fixed target region can be positioned in the vicinity of a releasemechanism or actuator, as at 44 in FIGS. 2-3, for causing a selectedreagent dispenser to dispense a desired reagent. Where more than one ofthe above components are employed, they will typically all be located inthe general area at or above the fixed target region. Details of suchcomponents are discussed more fully below.

Turning now to the reagent dispenser array, each reagent dispenser takesthe form of an elongated container, such as cylindrical or tubularcontainer 42 shown poised above array 16 in FIG. 1. The containers canbe formed, for example, of plastic, glass, and/or metal, or othermaterial. In one embodiment, each container is a rigid cylinder formedof a metal or metal alloy (e.g., aluminum, an aluminum alloy, orstainless steel), intended for repeated uses. In another embodiment,each container is constructed of a relatively inexpensive material, suchas glass or plastic that can be readily disposed of after its contents(reagent) have been exhausted.

By configuring each container with a sufficiently narrow diameter, ahigh density of such containers can be achieved. For example, variousembodiments contemplate from about 2 to 8 containers per cm², onaverage, or higher. One preferred embodiment contemplates an averagedensity of between about 3-6 containers per cm²; and most preferablybetween about 4-5 containers per cm². In an exemplary arrangement, aplurality of substantially like containers, each having a diameter ofless than 1cm, are disposed with substantially parallel longitudinalaxes and at closely spaced positions defining an array. In oneparticularly preferred embodiment, an array of such containers, eachhaving an outer diameter of about 4 mm, are arranged with acenter-to-center spacing between adjacent containers of about 4.50 mm.

Each container is provided with a passageway configured to receive andhold a respective reagent. In the embodiment of FIG. 1, a longitudinallyextending lumen, denoted as 42 a, holds a plurality ofreagent-containing ampules, such as 50. The passageway can be of anyhorizontal cross-section, such as circular, oval, polygonal, or othercross-section. Optionally, the exposed inner sidewalls of thepassageways can be covered with a substantially inert lining material.

A rack or frame, generally denoted as 46, provides a plurality ofholding cells, each being configured to support one reagent containertherein. In FIG. 1, for example, container 42 can be inserted into oneof holding cells 52 of frame 46 by lowering it in the direction of thedarkened arrow. The rack can have any number of holding cells. In oneembodiment, the number of different reagents held in the rack determinesthe number of holding cells. That is, there can be a one-to-onecorrespondence between the number of holding cells and the number ofdifferent reagents. For situations requiring a relatively large quantityof a particular reagent, other embodiments provide such reagent in twoor more holding cells of the array.

Rack 46 can have tens, hundreds, thousands, tens of thousands, orhundreds of thousands of holding cells. Advantageously, suchconfigurations permit the storage and variable selection of manydifferent reagents. In one particularly preferred embodiment, rack isformed with 10,000 holding cells, each removably supporting a respectivereagent container in a substantially upright fashion. In thisembodiment, any combination of up to 10,000 different reagents can bedispensed into the wells of a multi-well tray.

It should further be appreciated that a plurality of such racks can beutilized, for example, in an “assembly line” type arrangement. Forexample, three 10,000 cell racks can be arranged at respective locationsalong an automated system, each capable of dispensing up to 10,000different reagents.

The particular form of each reagent stored and dispensed in accordancewith the teachings of the invention is not critical, provided only thatit is compatible with the storage and dispensing means. The reagent,which can be a single substance or a grouping of different substances,can be provided, for example, as a solid, liquid, powder, emulsion,suspension or substantially any combination thereof. In one embodiment,a coating material is applied to a reagent core to form particulates,pills, beads or tablets. The coating can be dissolvable or swellable topermit access to the reagent under controllable conditions (e.g., uponexposure to a particular solvent).

Guidance for preparing coated micro-particles (beads) is provided, forexample, in: [1] R. Pommersheim, H. Lowe, V. Hessel, W. Ehrfeld (1998),“Immobilation of living cells and enzymes by encapsulation,” Institutfür Mikrotechnik Mainz GmbH, IBC Global Conferences Limited; [2] F. LimA. Sun (1980), Science 210, 908; [3] R. Pommersheim, J Schrezenmeir, W.Vogt (1994), “Immobilization of enzymes and living cells by multilayermicrocapsules” Macromol Chem. Phys 195, 1557-1567; and [4] W. Ehrfeld,V. Hessel, H. Lehr, “Microreactors for Chemical Synthesis andBiotechtechnology—Current Developments and Future Applications” in:Topics in Current Chemistry 194, A. Manz, H. Becker, MicrosystemTechnology in Chemistry and Life Science, Springer Verlag, BerlinHeidelberg (1998), 233-252; each expressly incorporated herein byreference.

In another embodiment, a plurality of bead-like particles act as solidsupports for the reagents. For example, reagents can be synthesized onthe beads, or absorbed thereto. In still a further embodiment, a slurryor dispersion comprised of a reagent and binding material is used toform a plurality of bead-like particles, with each individual beadhaving a substantially homogenous consistency.

A plurality of different reagents can be formed into respectivecollections or groups of reagent beads, or “lots.” For example, 10,000different reagents can be formed into 10,000 different bead lots, witheach lot comprised of a plurality of substantially like beads carrying arespective reagent. Beads from each lot can then be loaded intorespective dispensers of the dispenser array.

In one embodiment, a plurality of bead lots are formed, wherein eachbead includes a reagent core covered with a coating material, such as agelatin, having well-defined physical and chemical properties.Preferably, in this embodiment, all beads in all lots bear substantiallythe same outer coating (i.e., a “generic” coating). It should beappreciated that this arrangement reduces the risk of equipmentcontamination due to contact with the reagents. If any residues are leftbehind as the reagents move through the system, such residues will allbe of the same coating material. Preferably, the coating material ischosen so that any residues are innocuous to the system.

Further regarding reagent-carrying beads, the beads can be formed with adiameter slightly less than that of one of the passageways of thedispenser array, so that the beads can be stacked in each container, oneon top of the other, for gravity-fed dispensing. For example, beadshaving a diameter of between about 3.50-3.90 mm, and preferably about3.70 mm, can be stacked in a container having a passageway with adiameter of about 4 mm.

Alternatively, the beads can be relatively small, e.g., each having adiameter of less than about 1 mm. In one preferred embodiment, each beadhas a diameter of between about 275-325 μm, and preferably about 300 μm.A plurality of such beads can be placed in a capsule or ampule to bedispensed as a unit. For example, hundreds or thousands of beads fromthe same or substantially identical lots can be packed into plasticampules. The exemplary arrangement of FIG. 4 shows reagent-carryingbeads 62 disposed in bullet-shaped ampules 50. The various passagewaysin the dispenser array can be loaded with a plurality of such ampules,each containing beads from respective lots. As best seen in FIG. 4, theampules can be dimensioned to move downward through the passagewaysunder the force of gravity in a substantially single-file fashion.

The ampules can be provided with a cover member over an upper openingthereof. The cover member can be, for example, a removable cap or domehaving an open end configured to fit snugly about the upper region of anampule. Or, a frangible sheet-like film or membrane, such as membranes66 in FIG. 4, can be applied to an upper rim or lip surrounding theupper opening of each ampule. Access to the beads can be gained, forexample, by removing or rupturing the membrane cover.

In one embodiment, the cover over each ampule forms a substantiallyairtight seal, protecting the reagent contents of the ampule from theambient atmosphere. The seal can be effected, for example, usingconventional adhesives or by heating-sealing techniques. The sealedampules can further contain an inert gas, such as nitrogen or the like.

Substantially any reagent can be stored and dispensed using the systemof the present invention. According to one preferred embodiment, thereagent in each dispenser includes components useful for real timefluorescence-based measurements of nucleic acid amplification products(such as PCR) as described, for example, in PCT Publication WO 95/30139and U.S. patent application Ser. No. 08/235,411, each of which isexpressly incorporated herein by reference.

In one embodiment, each container holds an analyte-specific reagenteffective to react with a selected analyte that may be present in asample. For example, for polynucleotide analytes, the analyte-specificreagent can include first and second oligonucleotide primers havingsequences effective to hybridize to opposite end regions ofcomplementary strands of a selected polynucleotide analyte segment, foramplifying the segment by primer-initiated polymerase chain reaction.The analyte-specific detection reagent can further include afluorescer-quencher oligonucleotide capable of hybridizing to theanalyte segment in a region downstream of one of the primers, forproducing a detectable fluorescent signal when the analyte is present inthe sample.

An accession or tracking number can be printed on each container,identifying the reagent contained therein. For those embodimentsemploying ampules to hold the reagents, each ampule can bear such atracking number. With regard to the latter, the containers can be formedwith window regions through which the tracking numbers on the ampulescan be observed. The window regions can be of a transparent material,such as glass or plastic, or they can be openings or notches formed inthe sidewalls of the containers.

Preferably, each tracking number is provided in a machine-readableformat, such as a bar code. An operator can manually scan the bar codes,or they can be scanned in an automated fashion using robots. In oneembodiment, a robot picks up a container from a tray of reagent tubesand wands a bar code to learn and/or confirm the identity of the reagentheld therein. Using the scanned information, a control computerinstructs the robot to place the tube in a designated holding cell of arack.

Controllable dispensing of each reagent is provided by a gate mechanismlocated at a lower outlet region of each dispenser. Each gate mechanismis independently operable between (i) an opened condition permittingpassage of a respective reagent through the outlet region, and (ii) aclosed condition whereat such passage is blocked. The particularconstruction of the gate mechanism is not critical, provided only thatit is capable of retaining the reagent held in the respective containeruntil such time that it is desired to dispense the reagent.Additionally, each gate mechanism is preferably operable on anindividual basis, so that the various reagents can be dispensed one at atime.

Several exemplary gate mechanisms for use in connection with varioustypes of reagents will now be described.

According to one embodiment, each gate mechanism includes a magneticpinch valve having first and second magnets that are pivotally mountedin facing relation at a respective container outlet region. Generally,the pinch valve magnets have lower, confronting north and south poleregions that are urged toward one another by magnetic forces, therebynormally disposing the gate mechanism in a “closed” condition.Additional structure may be included to supplement or enhance suchnormal positioning of the pivotal magnets.

FIGS. 5 and 6 show one particular embodiment of a magnetic pinch valvethat is especially useful for dispensing reagent-carrying beads. Here, asupportive insert or plug, such as 170, is disposed in a lower region ofeach holding cell 152 of rack 146. Frictional engagement of the insert'souter sidewall with the inner sidewall of a respective holding cell canhold the insert in place. Adhesives or other retaining means may beemployed to ensure the long-term placement of each insert. The lower endof an elongated container 142, containing reagent beads 188, rests on anupper, inwardly flanged portion of insert 170 b. Insert 170 providespivot points, denoted as 174 and 176, on opposing inner sidewallportions to which respective permanent magnets 178, 180 pivotally attachat their midregions, e.g., by way of pivot pins. As best seen in FIGS.5(B) and 6(B), each pivotal magnet 178,180 is substantially C-shaped inhorizontal cross-section. Pivotal magnets 178,180 are oriented such thattheir upper and lower end regions are of opposite polarity. A thirdpermanent magnet, denoted as 184, is fixedly positioned along a sidewallportion of insert 170, above pivot points 174, 176. One end of thisstationary magnet 184 is disposed adjacent an upper region of one of thepivotal magnets, 178 or 180; and the other end of magnet 184 is disposedadjacent an upper region of the other pivotal magnet. Stationary magnet184 is oriented such that the polarity of each such end is opposite thatof the upper region of the pivotal magnet adjacent thereto. Accordingly,in the normal state, the upper region of each pivotal magnet 178, 180 isattracted to an adjacent portion of the sidewall-mounted magnet 184 and,at the same time, the confronting north and south pole regions of thepivotal magnets are attracted toward one another. In response, magnets178,180 pivot about their respective pivot points 174, 176 so that theirlower north and south pole regions swing toward one another; therebyassuming the closed condition, as shown in FIGS. 5(A) and 5(B). In theclosed condition, the outlet region is constricted such that the reagentbeads 188 are not able to fall out.

To release a reagent bead, a release mechanism is moved to a positionunder the outlet region of a selected container. The release mechanismis operable to overcome the closing force that normally prevents theegression of reagent beads. In the embodiment of FIGS. 5-6, anelectromagnet 192 is used as the release mechanism. Electromagnet 192has spaced-apart south and north pole portions disposed to attract theopposing north and south pole lower regions of pivotal magnets 178,180,respectively, in a direction away from one another. The magnetic forcegenerated upon activating electromagnet 192 is sufficient to overcomethe previously-described normal closing force, thereby swinging thelower regions of pivotal magnets 178, 180 apart so that one of the beads188 can fall through a central opening 170 a at the bottom of insert170.

As best seen in FIG. 6(A), upon swinging the lower regions of pivotalmagnets 178, 180 apart, the upper regions swing toward one another,thereby blocking the passage of any additional beads 188. Once a beadhas been dispensed, electromagnet 192 can be deactivated, permitting thegate mechanism to return to the closed position, as shown in FIGS.5(A)-5(B). Electromagnet 192 can then be moved to another container fordispensing another reagent. It should be appreciated that thisarrangement allows for the realization of controllable, single-beaddispensing.

FIGS. 7 and 8 show an embodiment of a pinch valve especially useful fordispensing a fluidic reagent. In this embodiment, two substantiallyplanar, permanent magnets 278, 280 attach at their uppermost ends to asupport member 270, for swinging motion about respective pivotalconnections 274, 276. As best seen in FIGS. 7(A) and 8(A), the pivotalconnections 274, 276 are disposed on opposing sides of a lowermostopening of elongated container 242. Support member 270, in turn, isfixed to a lower end region of container 242. In this regard, an annularcavity 287 extends upwardly from a lowermost rim or lip of container242, circumscribing longitudinal passageway 242 a. An upstanding.cylindrical collar 270 a, formed at the top of support member 270, isconfigured to fit snugly into cavity 287. Collar 270 a can be maintainedin cavity 287 by frictional forces and/or adhesive agents. A plate 272is secured against a lowermost end of holding cell 252 to provide alower foundation for supporting the container and gate assembly therein.

Gasket members 279, 281 (FIGS. 7(B) and 8(B)) are affixed to oppositeinner sidewall portions of support member 270. Gasket members 279, 281provide opposing planar surfaces positioned for sliding, substantiallyfluid tight, contact with the lateral side-edges of pivotal magnets 278,280. The opposing planar surfaces of gasket members 279, 281 can beformed of a hydrophobic material, and/or treated to exhibit hydrophobiccharacteristics, to discourage undesired leakage of the liquid reagent288.

Similar to the previous embodiment, pivotal magnets 278, 280 areoriented such that they having lower end regions of opposite polarity.So arranged, the lower ends of pivotal magnets 278, 280 are normallyattracted such that they swing toward one another and make contact,establishing a substantially fluid-tight seal (i.e., a “closed”position). In this regard, one or both magnets 278, 280 can bear apolymeric coating (not shown) along the region of contact to assist inthe formation of the fluid-tight seal.

To release the liquid reagent, a release mechanism is moved to aposition under the outlet region of a selected container. With referencenow to FIGS. 8(A)-8(B), an electromagnet 292 is employed as the releasemechanism. Electromagnet 292 has spaced-apart south and north poleportions disposed to attract the opposing north and south pole lowerregions of pivotal magnets 278, 280, respectively, in a direction awayfrom one another. The magnetic force generated upon activatingelectromagnet 292 is sufficient to overcome the previously-describednormal closing force, thereby swinging the lower regions of pivotalmagnets 278, 280 apart so that an aliquot of fluidic reagent 288 canfall through a central opening 272 a in plate 272. The duration ofactivation of electromagnet 292 can be used to gauge the amount ofliquid reagent dispensed. When electromagnet 292 is turned off, thenormal attraction between the opposing lower end regions of magnets 278,280 returns the valve to a closed position.

In another embodiment, each gate mechanism is a resiliently deflectablelever having a protrusion that normally extends into a respective outletregion. In an exemplary arrangement, as illustrated in FIGS. 9(A)-9(B),an elongated lever, indicated generally by the reference numeral 302,extends longitudinally along the outer sidewall of container 342,proximate the container's lower outlet region. Upper and lower nubs orprotrusions, denoted respectively as 302 b and 302 c, project outwardlyfrom one side of lever 302, towards container 342. Container 342, inturn, is provided with upper and lower bores, indicated respectively as342 b and 342 c, that extend fully through its sidewall at locationsadjacent to lever 302. More particularly, each of bores 342 b, 342 c ispositioned in alignment with a respective one of protrusions 302 b, 302c, and is configured to removably receive such protrusion therein.

A resilient spring member, such as leaf spring 351, is secured at one ofits ends to a support structure 353 that is fixedly positioned near thelower outlet region of container 342. The other end of leaf spring 351is disposed to act against a side of lever 302, opposite container 342.Leaf spring 351 provides a normal biasing force, along the direction“F,” that presses the lower end region of lever 302 against container342. Under these circumstances, lower protrusion 302 c extends throughlower bore 342 c and into passageway 342 a at the container's loweroutlet region, as shown in FIG. 9(A). In this “closed” position, lowerprotrusion 302 c blocks the egression of any reagent-carrying ampules388 a-388 c. Upper protrusion 302 b, on the other hand, is positionedoutside of passageway 342 a in the normal, closed position.

To release an ampule, a release mechanism is moved to a position underthe outlet region of a selected container. The release mechanism isoperable to overcome the closing force that normally prevents theegression of ampules. With reference to FIGS. 9(A) and 9(B), anelongated rod 372 can be employed as the release mechanism. Rod 372 isadapted for reciprocal linear motion between (i) a retracted position,whereat rod 372 is positionable below a selected lever, such as lever302 in FIG. 9(A), to (ii) an extended position, whereat a rounded-headportion of rod, denoted at 372 a, can abut and press against a lower,outwardly-angled cam surface of lever, as at 302 a in FIG. 9(B), therebydeflecting lever 302 away from container 342. Such motion of rod 372 canbe effected, for example, by providing rod at the end region of amovable plunger extending from a conventional solenoid assembly.

Notably, when lever 302 is deflected in the manner just described, lowerprotrusion 302 c is withdrawn from passageway 342 a, permitting thebottommost reagent-carrying ampule 388 a to fall from the container'slower outlet region. Also during such deflection, upper protrusion 302 bis received within upper bore 342 b such that it extends into passageway342 a, preventing the egression of any remaining ampules 388 b-388 c.Upon returning rod 372 to its retracted position, lever 302 reassumesits normally closed position, as in FIG. 9(A), preventing the passage ofany ampules. It should be appreciated that this arrangement permitscontrollable, single-ampule dispensing.

In an alternative embodiment, similar to the embodiment just described,the release mechanism operates according to magnetic principles. In anexemplary arrangement (not shown), the lower end of the resilientlydeflectable lever and the upper head portion of reciprocally movable rodare magnetically polarized, or polarizable, to exhibit the same polarity(e.g., both being “N”). The lever can be deflected by moving the rodinto proximity with the lever's lower end, so that the like magneticpole portions repel one another. Notably, contact between the rod andthe lever is not required in this embodiment. The magnetic repulsion issufficient to deflect the lever away from the container, therebypermitting a reagent-carrying ampule to fall from the container's loweroutlet region.

Any of the above-described release mechanisms can be adapted forvariable positioning along a generally horizontal plane under thedispenser array by mounting it to the upper surface of the lower xystage. In this regard, a mounting assembly, such as bracket 51 of FIGS.1-3, can be used to hold the release mechanism. In this embodiment, oneend of bracket 51 is affixed to the upper surface of lower xy stage 26.The other (free) end of bracket 51 extends into the region between theplatform assembly 12 and the dispenser array 16, whereat the releasemechanism is supported, as schematically indicated at 44.

A detection assembly can be provided for detecting the passage ofreagent into a receptacle from a selected overhead dispenser. In theexemplary arrangement of FIGS. 1-3, the region between a radiationsource or emitter, such as laser 37, and a radiation sensor, as at 39,defines a detection zone. In this embodiment, both laser 37 and sensor39 are fixedly positioned at respective locations on the upper surfaceof the lower xy stage 26, on opposing sides of upper xy stage 22. Thisconstruction permits movement of the detection zone along a generallyhorizontal plane under dispenser array 16 with movement of the lower xystage 26. Thus, by moving xy stage 26 in an appropriate manner, thedetection zone can be placed under any selected dispenser of array 16. Anarrow-width beam, indicated at 38, can be directed from laser 37 alongthe detection zone and detected by sensor 39. An interruption in thebeam 38 indicates the passage of a reagent from a dispenser above thedetection zone.

Lasers and sensors, suitable for use in practicing the invention, areavailable commercially, for example, from Edmund Scientific (Barrington,N.J.). A particularly preferred diode laser, for use as a radiationemitter, is made by Coherent, Inc. (Auburn, Calif.).

In addition to detecting the passage of reagent, beam 38 can also beemployed to confirm the identity of a dispensed reagent. For example,beam 38 can “double” as a bar-code scanner. In one embodiment, eachreagent-carrying ampule bears a bar code that is unique to theparticular type of reagent held therein. As a dispensed ampule passesthrough the beam, the bar code is read and the information is passed onto the control computer. The computer can then positively identify thedispensed reagent, and take appropriate corrective measures in the eventof a dispensing error.

A guide or funnel member can be provided in the region between thedispenser array and the platform assembly for channeling reagentdispensed from an overhead dispenser to a selected site on thereceptacle-holding area of the upper xy stage. In the exemplaryarrangement of FIGS. 1-3, such a guide member, indicated schematicallyat 40, is fixedly positioned relative to the upper surface of lowersupport by way of mounting bracket 51. This construction permitsmovement of the guide member 40 along a generally horizontal plane underthe dispenser array 16 with movement of the lower xy stage 26.

Typically, in operation, guide member 40 will be positioned under aselected dispenser of array 16. A selected receptacle, such as aparticular well of multi-well plate 36, will be positioned under theguide member by appropriately moving the upper xy stage 22. Suchpositioning of the guide member and the receptacle will preferably occursimultaneously. A dispensed reagent, then, will fall through a central,vertically extending channel of guide member 40 on its way from theselected dispenser to the selected receptacle.

In one particular embodiment, shown in FIGS. 9(A) and 9(B), a guidemember, denoted as 340, includes (i) an upper opening 340 a, (ii) alower opening 340 c, smaller than the upper opening 340 a, and (iii) aconical or funnel-shaped portion 340 b, between the upper and loweropenings. Also in this embodiment, it should be noted that the guidemember 340 and reciprocally-movable rod 372 are conveniently providedtogether in a common housing.

At this point, the significance of the fixed target region of the lowersupport can be well appreciated. The fixed target region is primarily areference point that, when positioned under a selected dispenser,facilitates the proper and simultaneous positioning of one or moreadditional components thereunder. For example, with reference to theembodiment of FIGS. 1-3, each of the following elements is positioned atan appropriate location with respect to a selected container, asdescribed, upon moving the fixed target region 26 a to a location undersuch container:

(i) radiation beam 38 is located under the selected container so thatany reagent dispensed from the container will break the beam;

(ii) guide or funnel member 40 is disposed with its upper, large openingaxially aligned with the lower outlet region of the selected containerso that dispensed reagent will fall therein; and

(iii) release mechanism 44 is positioned proximate a normally closedgate mechanism at the lower outlet region of the selected container.

Moreover, a selected well of multi-well plate 36, supported at thereceptacle-holding area of the upper xy stage 22, can be moved to aposition over the fixed target region while the fixed target region isbeing moved under the selected receptacle as just described. In thisway, the receptacle, too, can be quickly and accurately positioned toreceive a dispensed reagent.

As with the xy stages 22, 26, operation of the various components andsub-assemblies described above can be controlled and orchestrated usingthe LABVIEW® or LABWINDOWS® software from National Instruments (Austin,Tex.) by techniques known in the art.

In a typical use, an array of holding cells 52 in rack 46 are loadedwith respective containers 42, each holding a particular reagent. A dataset or table is created comprised of values identifying each location oraddress of the holding-cell matrix with its particular resident reagent.The data set is stored electronically on a drive unit accessible to acontrol computer. The reagents are maintained in rack 46 untildispensed, e.g., as follows.

Multi-well plate 36 is placed on receptacle-holding area 22 a of upperxy stage 22. A plurality of reagents, stored in rack 46, are selectedfor dispensing into chosen wells 32 of plate 36, and this information isfed to the control computer. The computer accesses the data set oflocation information to determine which containers hold the selectedreagents, and a loading sequence is constructed and held in memory. Thecomputer signals the motor controller to move lower xy stage 26 to alocation whereat its fixed target region 26 a is disposed under thefirst container of the loading sequence. At the same time, upper xystage 22 positions a selected target region of the receptacle-holdingarea 22 a, underlying a chosen well of multi-plate 36, over the fixedtarget region 26 a. Together, these steps serve to position radiationbeam 38, guide member 40, and release mechanism 44 at appropriaterespective locations proximate a lower outlet region of the container,as well as to position the chosen well of plate 36 under the container.The computer then signals activation of release mechanism 44, causingthe gate mechanism at the lower outlet region of the container to openand dispense an aliquot of its respective reagent into the chosen wellof plate 36. A correctly dispensed substance will briefly preventradiation beam 38 from reaching sensor 39, indicating successfuldispensing. If radiation beam 38 is not interrupted, as expected, anerror in dispensing is recorded and a further attempt at dispensing thereagent can then be made, as desired. After the first reagent has beendispensed, the next reagent of the loading sequence can be dispensed ina like manner.

In one preferred embodiment, the dispensing system of the presentinvention is utilized as a sub-assembly in a larger system for loadingan array of very small wells in a microcard with respective reagents. Inthe exemplary arrangement of FIG. 10, a 384-well plate, denoted as 436,serves as a source of reagent for serially loading target 384-wellmicrocards, such as 441, supported for movement on a carousel assembly449. The wells of both the plate and the microcards are disposed in16×24 arrays, spaced about 4.5 mm center-to-center. The wells of theplate, however, have a greater diameter than the wells of themicrocards. For example, each well of the plate can be configured with adiameter of about 3 mm, while each well of the microcards can be formedwith a diameter of about 1 mm.

Generally, each well of the multi-well plate 436 is loaded with arespective reagent-carrying ampule, in accordance with the foregoingdetailed description. A robot, such as 443, then moves the ampule-loadedplate, in the direction of darkened arrow “P,” to a pick-and-place unit445. Pick-and-place unit 445 simultaneously retrieves a reagent beadfrom each ampule in the plate, and retains the beads at spaced-apartlocations defining an array corresponding to the plate and microcardarrays. Pick-and-place unit 445 then rotates about its central axis toposition the retained beads over the wells of a microcard supported onthe carousel at a location directly adjacent thereto, at which point itreleases the beads. A bead distributor, such as 447, interposed betweenpick-and-place unit 445 and the target microcard, separately channelseach released bead into its designated well. The loaded microcard wellscan then be used to carry out a desired assay or reaction, such asreal-time PCR.

Additional details of the micro-card loading system are provided in copending application Atty. Docket No. 0550-0076/4424, filed concurrentlyherewith and expressly incorporated herein by reference.

The many benefits offered by the storage and dispensing system of thepresent invention can now be appreciated. For example, simultaneousmovement of the upper and lower supports (e.g., xy stages) allows quickand accurate positioning of the various system components. Moreover,once a desired receiving receptacle is positioned under a selecteddispenser, the release mechanism and detection beam can operateimmediately to dispense the substance. Consequently, serial dispensingof a plurality of substances can be accomplished in a very rapid manner.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular embodiments and examplesthereof, the true scope of the invention should not be so limited.Various changes and modification may be made without departing from thescope of the invention, as defined by the appended claims.

1. A system for storing and dispensing a plurality of solid reagents,comprising: an addressable array of solid reagent dispensers; a gatemechanism at a lower outlet region of each dispenser, each gatemechanism being independently operable between (i) an open conditionpermitting passage of a respective solid reagent through said outletregion, and (ii) a closed condition whereas such passage is blocked; afirst support disposed below said array; and a second support mounted onsaid first support, said second support having a holding area forreceiving a plurality of receptacles; wherein (i) said first support isvariably positionable, permitting placement of a fixed target regionthereof directly under any selected one of said dispensers in saidarray, and (ii) said second support is variably positionable, permittingplacement of any selected target site of said holding area directly oversaid fixed target region.
 2. The system of claim 1, wherein said firstand second supports are independently operable xy stages.
 3. A systemfor storing and dispensing a plurality of solid reagents, comprising: anaddressable array of solid reagent dispensers; a gate mechanism at anoutlet region of each dispenser, each gate mechanism being independentlyoperable between (i) an opened condition permitting passage of arespective solid reagent through said outlet region and (ii) a closedcondition wherein said passage is blocked; a first support disposedadjacent said array; and a second support mounted on said first support,said second support having a holding area for receiving a plurality ofreceptacles, wherein (i) said first support is variably positionable topermit placement of a fixed target region thereof in alignment with anyselected one of said dispensers in said array, and (ii) said secondsupport is variably positionable to permit placement of any selectedtarget site of said holding area in alignment with said fixed targetregion.
 4. The system of claim 3, wherein said dispensers are elongatedcontainers, each having a longitudinally extending passageway configuredto receive and hold a respective solid reagent when said gate mechanismis in the closed condition.
 5. The system of claim 4, further comprisinga rack having an array of at least about 100 holding cells, each holdingcell being configured to removably support one of said containers in asubstantially upright fashion.
 6. The system of claim 5, wherein saidholding cells are disposed at an average density of at least about 3holding cells per cm².
 7. The system of claim 6, wherein said arrayincludes at least about 500 holding cells; and wherein said holdingcells are disposed at an average density of at lest about 4 holdingcells per cm².
 8. The system of claim 7, wherein said array includes atleast about 1,000 holding cells.
 9. The system of claim 8, wherein saidarray includes at least about 10,000 holding cells.
 10. The system ofclaim 9, wherein said array includes at least about 100,000 holdingcells.
 11. The system of claim 4, further comprising a plurality ofdifferent solid reagents disposed in said dispenser passageways.
 12. Thesystem of claim 11, wherein each of said passageways contains a solidreagent that is unique to said array.
 13. The system of claim 11,further comprising a plurality of solid reagent bead lots, each lotcomprised of a plurality of substantially similar reagent beads andmoveable through said outlet region.
 14. The system of claim 13, whereineach reagent bead has a diameter of less than about a millimeter. 15.The system of claim 11, wherein the plurality of different solidreagents comprises particulate reagents.
 16. The system of claim 11,wherein the plurality of different solid reagents comprises powderreagents.
 17. The system of claim 11, wherein the plurality of differentsolid reagents comprises beads.
 18. The system of claim 3, wherein saidfirst and second supports are independently operable xy stages.
 19. Thesystem of claim 3, wherein each gate mechanism is subject to a normalbiasing force that urges it to the closed position, thereby preventingthe passage of a solid reagent through a respective outlet region. 20.The system of claim 19, further comprising a release mechanismpositionable near any one of said gate mechanisms and operable to applya secondary force of a magnitude and direction effective to override thenormal biasing force so that the gate mechanism assumes the openedcondition.
 21. The system of claim 20, wherein each gate mechanism is aresiliently deflectable lever having a protrusion normally extendinginto a respective outlet region.